Methods:Our multi-criteria optimization (MCO) IMPT planning system (ASTROID) allows assigning separate target volume for each beam, such as in split target plans. For pediatric chordoma cases, a variety of target sub-volumes (TSV) were generated by 4mm and 8mm uniform constriction of the original clinical target volume (CTV), and their subtraction from CTV and each other, forming a co-centric shell structure arrangement. In addition to the original plan, utilizing three beams all assigned to the full CTV, five plans were optimized for the same beam angles but different assigned TSV combinations. To increase the degrees of freedom the original beams were duplicated to allow for TSVs to be assigned to multiple beam-angles, having a total of up to 9 beams. All plans utilized all three beam angles and single dose prescription to CTV. Dose and linear energy transfer (LET) distributions were calculated by TOPAS (TOol for PArticle Simulation) Monte Carlo system and evaluated via dose- and LET- volume histograms and a relative biological effectiveness (RBE) model.

Results:While dose variations for the CTV were negligible, mean LET value deviations of more than 30% were noticed among the different plans. For the organs at risk (OARs), variations in dosimetric, LET and RBE values were more prominent. The amount of variation among the plans seemed to be dependent on their volume and location relative to the target.

Conclusion:We show that the definition of sub-targets at the treatment plan optimization level may impact the biological effectiveness of proton treatments.